The present invention relates to a monitor for the instrument-landing system as set forth in the preamble of claim 1. A monitor of this kind is described in an article by H. Rausch, "Moderne Funk-Landesysteme", 6th Vol., 1974, No. 11, pages 223 to 227.
The radiation of the localizer and glide-path signals is monitored by a monitor located about 100 m from the antenna array. When the tolerances specified by the ICAO (International Civil Aviation Organization) are exceeded in the monitor, an alarm signal is provided which either turns the transmitter off or switches it to a standby transmitter.
The following description will be concerned only with the localizer signals. On the runway center line, the difference in depth of modulation (DDM) between the 90-Hz and 150-Hz patterns of the localizer signal must be zero. Any deviations therefrom, which are detected in the monitor, may be caused both by equipment errors and by overflying aircraft. Since, however, the alarm signals are to be provided only in the presence of equipment errors, it must be determined whether the deviation from the specified DDM value is caused by an overflying aircraft or not. To this end, conventional prior art installations employ a second monitor, e.g., a near-field monitor. The signals received by this monitor are not disturbed by overflying aircraft. Thus, an equipment error is identified only if both monitors indicate an error.